Infection with HIV-1 is characterized by a decrease in function and number of CD4+ T cells which are required for helper function in the antibody response to protein antigens. We have previously shown that antibody responses to HIV-1 proteins in mice are T-helper cell dependent, whereas HIV proteins, conjugated to Brucella abortus (BA) do not require CD4+ T cells. Subsequently, we conjugated a 13 amino acid peptide (H-A-A-I-G-P-G- R-A-F-T-A-C) derived from the third variable region of HIV-MN envelope (bold type) to BA (V3-BA), to keyhole limpet hemocyanin (V3-KLH) and to ficoll (V3-ficoll). All three conjugates (V3-BA, V3-KLH and V3-ficoll) were immunogenic in normal mice. IgG1 was the predominant isotype induced by V3-KLH, whereas IgG2a was the pre-eminent IgG subclass elicited. Unlike V3-BA and V3-KLH, V3-ficoll failed to elicit responses in X-linked immunodeficient mice. These results are analogous to anti-TNP antibody responses previously obtained using the same carriers (BA, KLH and ficoll) but with TNP as the hapten, suggesting that V3-BA behaves as a T- independent type 1 antigen. Currently, we are studying the response to the V3-conjugates in mice depleted of CD4+ T cells. In addition, sera from the mice immunized with the V3-conjugates were tested for ability to neutralize different HIV-1 isolates. Isolates which retain the G-P-G-R-A-F, namely, IIIB, MN and SF2 were neutralized (greater than or equal to 50% syncytia inhibition) by sera from V3-BA immunized mice at titers of 160 or greater. The RF isolate, however, which is different at this site (G-P-G-R-V-I) was neutralized at a lower titer (20). In contrast sera from mice immunized with gp120-BA were capable of neutralizing all four isolates at a titer of 160 or higher. This was probably due to the fact that sera from gp120-BA- immunized mice, but not from V3-BA-immunized mice, were capable of blocking soluble CD4 binding to gp120 on HIV-1-infected cells (determined by flow cytometry). These results indicate that B. abortus can be used as a carrier in the development of subunit vaccines designed to generate anti- HIV responses.